The present invention generally relates to vehicle rearview mirror assemblies, and more particularly relates to rearview mirror assemblies that incorporate some component of an electronic compass.
It is known to associate compass systems with rearview mirrors in vehicles. FIG. 1 illustrates an example of a known rearview mirror assembly incorporating such a compass system. The mirror assembly 10 comprises an open-faced mirror housing 12, a mirror 14, and a circuit board 16. Mirror 14 includes an electrochronic element driven by a driving circuit 18 for automatically changing mirror reflectance as a function of glare. It will be understood that, as typically, mirror 14 can be a simple plain mirror with a prism mechanism (not shown) to enable reduction of nighttime glare.
Housing 12 is mounted to the vehicle by way of a pivot connection 20 and a bracket housing 22. In FIG. 1, mirror housing 12 is shown mounted to bracket housing 22 on a windshield 24 of the vehicle as in the case of an interior rearview mirror assembly. Pivot connection 20 shown in FIG. 1 comprises a ball 26 fixedly mounted to mirror housing 12 and pivotally mounted in a socket of a spacer 28. The other end of spacer 28 carries a socket which receives another ball 30 which is fixedly secured to bracket housing 22. It can be seen that the typical mirror housing 12 is thus pivotable relative to the vehicle windshield 24 by manually rotating either of the balls 26, 30 in their respective sockets. It is also known to use a single ball and socket pivot to mount a mirror housing to a vehicle. Further, it is known to incorporate a motorized actuating mechanism in the housing 12 wherein the housing can be pivoted relative to the vehicle by remote actuation of a switch.
The compass system of FIG. 1 comprises a magnetic field sensor 32 mounted in bracket housing 22 for detecting the earth's magnetic field and for providing electrical signals representative of the magnetic field to circuit board 16 through a wire 34 to circuit board 16. Connector 36 on circuit board 16 extends into an aperture 37 in mirror housing 12 to deliver power and signals to and from other mirror system components. Wire 34 extends through a notch in aperture 37 to a connector 45 on circuit board 16. Circuit board 16 carries a processing circuit 38 that processes signals from sensor 32 and provides a representative output signal to a display 40 that displays the vehicle's heading. In the example of FIG. 1, display 40 can be visually seen by an observer through a transparent window 42 in mirror 14. The display may be mounted either behind the mirror element or in an area adjacent to and offset from the mirror element. Alternatively, the display may be located elsewhere in the vehicle, such as an overhead console, instrument panel, A-pillar, etc.
Magnetic field sensor 32 comprises an X-axis sensor 44, perpendicular to the vehicle's direction of travel, and a Y axis sensor 46, in line with the vehicle's direction of travel. Moreover, both sensors 44, 46 are mounted parallel with the earth's surface. In other words, magnetic field sensor 32 is fixed relative to the vehicle. Magnetic field sensor 32 is typically a flux-gate sensor, although other types of magnetic field sensors are in known use, such as magneto-resistive and magneto-inductive sensors.
In such constructions, the compass sensing circuit elements that sense magnetic field components of the earth's magnetic field are typically mounted a separate housing attached to a foot portion of the mirror mounting bracket or in another area of the vehicle altogether, such as the overhead console. Mounting the sensors in the foot of the mirror mounting bracket has been preferred due to the low levels of magnetic interference that may be present in other vehicle locations. The problem with mounting magnetic sensors at the foot of the mirror mounting bracket is that the foot portion becomes much larger and hence may block a more significant amount of the driver's field of view through the windshield. Also, for electrochromic mirrors, a large bracket housing reduces the light level reaching the ambient sensor aperture in the mirror housing. Another problem is that the foot portion of the mirror may be more suitable for mounting of other components, such as rain sensors or a GPS antenna as disclosed in commonly-assigned U.S. patent application Ser. No. 09/250,086.
Because the compass display is commonly mounted in the mirror housing, it has typically been desirable to also mount the compass processing circuit in the housing along with the display and its associated circuitry. Thus, it is also beneficial to mount the compass magnetic field sensors in close proximity to the compass processing circuit so as to avoid the need of running excess wiring between the sensors and the processing circuit. Moreover, remote mounting of the magnetic field sensors from the processing circuit introduces noise into the system due to electromagnetic interference (EMI) picked up by the wire harness from other nearby electronic circuits. To reduce the EMI noise introduced between the sensors and the processing circuit, EMI filters are required. Also, the wire harness itself tends to radiate EMI emissions that may introduce noise to the vehicle's radio. The wire harness, connectors, sensor housing, and EMI filters all add cost to the mirror itself while also increasing the complexity and cost of installing the mirror assemblies in vehicles.
While the inside of the mirror housing would appear to be the most desirable location for the sensors so as to provide all the components of the compass in a single location while also freeing up the area at the foot of the rearview mirror mounting bracket for other components, the mounting of the compass sensors in the rearview housing has presented major problems that have a significant impact upon the accuracy of the compass. Specifically, rearview mirror housings are mounted to a vehicle so as to be both vertically and horizontally pivotable about one or more universal ball joints provided on the mounting bracket or on the back of the mirror. Such pivotable movement is necessary so as to accommodate drivers of different sizes and to allow drivers to adjust the mirror in accordance with any adjustments in their seat position so as to have a clear view out the rear window of the vehicle. Such movement of the mirror housing correspondingly causes movement of the magnetic field sensors of the compass sensing circuitry. Such movement of the compass sensors can produce a rather significant error in the vehicle heading information processed by the processing circuit and displayed to the driver.
While electronic compass circuits are known that continuously recalibrate themselves (see, for example, U.S. Pat. Nos. 4,953,305, 5,737,226, 5,761,094, and 5,878,370, such systems do not compensate for all the errors that may be introduced through the horizontal and vertical tilting of the mirror housing. To the extent these mirror circuits may correct for some of the errors that may be introduced in this manner, these circuits are specifically programmed to not respond to an immediate change in detected field strength, but rather are programmed to recalibrate only after either a predetermined time period or predetermined other conditions are sensed. In this manner, these electronic compasses may recalibrate to account for changes in the vehicle magnetic field that may be caused by aging or other post manufacture influences such as magnetic roof-mounted antennas, while ignoring many temporary changes to the magnetic field sensed by the compass sensors that are not caused by a change in vehicle direction. Such temporary, but significant, fluctuations in the magnetic field readings occur when the vehicle passes by objects having large amounts of ferrous material, such as railroad tracks, bridges, and large buildings or when the vehicle moves through a car wash. Accordingly, such circuits are programmed to ignore or otherwise have a slower response time to many temporary field variations that they may sense. Thus, these circuits, if placed in a rearview mirror housing, would not immediately respond to any variations sensed as a result of movement of the mirror housing and sensor elements. On the other hand, if these compass processing circuits do not ignore or were more responsive to any abrupt field variations, they would frequently generate inaccurate vehicle heading readings when only temporary field variations are present.
Additionally, the prior art electronic compass circuits described above, generally do not instantaneously recalibrate. Recalibration normally requires that the vehicle travel through one or more 360 degree loops or, in the case of U.S. Pat. No. 5,737,226, at least travel through a turn of some significant degree.
In U.S. Pat. No. 6,023,229, a variety of different compensation mechanisms are proposed that would allow for compensation of horizontal tilting of the rearview mirror assembly when the sensor elements are mounted in the housing. In general, compensation for horizontal tilting is accomplished by adding or subtracting a predefined error value to numerical heading otherwise computed by the processing circuit and used to determine which of the eight-point headings (N, NE, E, SE, S, SW, W, NW) to display. The reason this approach is effective is illustrated in FIGS. 2-4. Referring to FIGS. 2 and 3, for purposes of illustration, the Y axis is defined as the axis aligned with the direction of the vehicle, the X axis is defined as the axis perpendicular to the Y axis and parallel with the earth's surface, and the Z axis is defined as the axis perpendicular to the X and Y axes.
When the two magnetic field sensors of the compass are fixedly mounted to the foot portion of the mirror mounting bracket that is attached to the vehicle windshield, one of the sensors is permanently mounted to detect magnetic field components along the Y axis while the other is permanently mounted to detect magnetic field components along the X axis. The two sensors would have the same relation to the X and Y axes of the vehicle when mounted with the X axis sensor parallel to the mirror and the Y axis sensor perpendicular to the mirror surface provided that the mirror surface lies in a plane P1 that is parallel to the X axis. Thus, if no magnetic field component is sensed by the X axis sensor and a positive magnetic field component is sensed by the Y axis sensor, the compass processing circuit would determine that the vehicle is headed north. Similarly, if no magnetic field component is sensed by the X axis sensor and a negative magnetic field component is sensed by the Y axis sensor, the compass processing circuit would determine that the vehicle is headed south. Likewise, if no magnetic field component is sensed by the Y axis sensor and a positive magnetic field component is sensed by the X axis sensor, the compass processing circuit would determine that the vehicle is headed east. If no magnetic field component is sensed by the Y axis sensor and a negative magnetic field component is sensed by the X axis sensor, the compass processing circuit would determine that the vehicle is headed west. If equal positive magnetic field components are sensed by both the X and Y axis sensors, the compass processing circuit would determine and that the vehicle is headed northeast. If equal negative magnetic field components are sensed by both the X and Y axis sensors, the compass processing circuit would determine and that the vehicle is headed southwest. If a positive magnetic field component is sensed by the Y axis sensor that is equal to the absolute value of a negative magnetic field component sensed by the X axis sensor, the compass processing circuit would determine that the vehicle is headed northwest. If the absolute value of a negative magnetic field component that is sensed by the Y axis sensor is equal to the value of a positive magnetic field component sensed by the X axis sensor, the compass processing circuit would determine that the vehicle is headed southeast. Because the magnitude of the earth's magnetic field as sensed in a horizontal plane generally does not change for a given location on the earth, the component values of the earth's magnetic field (B.sub.E) as sensed in the X and Y axes (B.sub.X and B.sub.Y, respectively) may be determined using Pythagoreans's Theorem, B.sub.E.sup.2 =B.sub.X.sup.2 +B.sub.Y.sup.2 (where the magnetic fields are generally measured in milliGuass (mG)). Thus, if the output levels of the magnetic sensors were plotted relative to the X and Y axes as the vehicle turned through a 360 degree loop, the levels relative to one another would form a circle as depicted as circle A in FIG. 4.
Because such electronic compasses generally only display eight different headings (N, NE, E, SE, S, SW, W, and NW) and because the magnetic field components sensed by the X and Y axis sensors are not always zero and are not always equal, the compass processing circuit generally computes a heading angle .theta. relative to the X and Y axes, and compares this heading angle to thresholds that define the boundaries between each of the eight different heading displays. Thus, the circular plot A shown in FIG. 4 is effectively split into eight angular segments of 45 degrees corresponding to the eight different display headings. The compass processing circuit simply determines in which segment heading angle .theta. lies to determine which of the eight headings to display.
When the rearview mirror is horizontally tilted such that the plane of the mirror lies in plane P (Pig. 2), the X and Y axes of the vehicle no longer correspond to the axes of the sensors. Instead, the X' and Y' axes of the sensors are rotated about the origin of the X and Y axes of the vehicle by the angle .phi.. The effect of such a rotation is that the compass processing circuit would compute vehicle heading angles that are incorrect by an amount equal to angle .phi.. Ideally, the exact angle .phi. that the mirror is tilted could be determined and be compass processing circuit could simply add or subtract angle .phi. to or from the sensed vehicle heading angle .theta.. U.S. Pat. No. 6,023,299 discloses that the relative angle .phi. that the mirror is tilted could be sensed using microswitches or a signal could be obtained from a motorized mechanism for rotating the mirror if such a mechanism is present. Alternatively, U.S. Pat. No. 6,023,229 discloses that a predetermined mirror horizontal tilt angle .phi. could be programmed into the compass processing circuit regardless of the actual horizontal tilt angle due to most American original equipment manufacturers' error tolerances for electronic compasses and due to the fact that the compass only display is one of eight possible vehicle headings rather than the exact heading in degrees. For example, it has been determined that in a sample of different sized drivers and different sized vehicles, the typical actual horizontal tilt angle .phi. is between 15 and 21 degrees. Most American original equipment manufacturers will accept a typical error of .+-.10 degrees for generating the final display readout. By setting the predetermined horizontal tilt angle .phi. to 18 degrees, this predetermined angle will only differ from the actual horizontal tilt angle by .+-.3 degrees. When coupled with other errors inherent in such electronic compass systems, the .+-.3 degree error resulting from horizontal tilting does not produce errors outside of the manufacturers' .+-.10 degree error tolerance.
While the techniques described above are effective for compensating for horizontal tilting, they do not compensate for vertical tilting of the housing. Vertical tilting of the rearview mirror housing is very significant in view of the fact that in some geographic areas, the earth's magnetic field strength component in the vertical (or "Z direction") is much stronger than its horizontal component that points towards the earth's magnetic north pole. Accordingly, when the X and Y axis magnetic sensors of an electronic compass are tilted vertically, a vertical component of the earth's magnetic field is sensed in the X and Y sensors thereby causing a significant shift in the magnetic field strength sensed by the compass sensing circuit. FIG. 5 shows the effect of tilting the mirror vertically. Specifically, circle B represents the plot of sensor output levels prior to tilting, and circle C represents the plot of sensor output levels following vertical tilting. As evident from FIG. 5, vertical tilting of the mirror and sensors causes the entire circular plot to shift along the Y axis by an amount equal to the portion of the Z component of the earth's magnetic field that is sensed by the Y axis sensor. Such shifting introduces significant errors when the vehicle is headed east or west. As apparent from the above discussion pertaining to horizontal tilting, the added error introduced by vertical tilting may exceed the manufacturers' error tolerances.
U.S. Pat. No. 6,023,229 does disclose at least one possible method for compensating for vertical tilting of the mirror and compass sensors. Specifically, it discloses first detecting whether a vertical tilt has occurred by providing an additional magnetic field sensor oriented in the Z axis, and by monitoring both the resultant of the X, Y, Z vector and the X,Y magnitude. If the resultant of the X, Y, Z vector does not change but the X,Y magnitude suddenly changes, it is deemed likely that the mirror has been vertically tilted. When such a vertical tilt is detected, the processing circuit may perform the quick recalibration routine disclosed in U.S. Pat. No. 5,737,226, which essentially repositions the origin of the reference X and Y axes to correspond to the center of the circular plot of data obtained from the sensors as the vehicle travels in 360 degree loops.
While the methods for compensating for horizontal and vertical tilt of the rearview mirror and sensors described in U.S. Pat. No. 09/260,267 are effective, the need to use an additional magnetic field sensor for sensing Z axis field components to correct for vertical tilting, not only adds to the cost, but also adds to the bulk of the components that are mounted in the rearview mirror housing. Additionally, the reliance on the compass processing circuit to recalibrate the compass after vertical tilting has been detected may introduce temporary erroneous readouts at least until the vehicle travels through a sufficient turn to enable recalibration.
Therefore, there exists a need for a mechanism for quickly and accurately compensating the electronic compass for any vertical tilting of the rearview mirror assembly housing. There also exists a need for a mechanism for compensating the electronic compass for any vertical tilting of the rearview mirror assembly housing without adding significantly to the cost of the mirror assembly.